The present invention relates to the detection of aberrations associated with optical systems (such as the projection system and/or radiation system) utilized in a lithographic projection apparatus, and more particularly to the design, layout and application of aberration monitoring structures that can be used to monitor the optical system performance during the manufacture of semiconductor (and other) devices using such apparatus. A lithographic projection apparatus generally comprises:
a radiation system for supplying a projection beam of radiation;
a support structure for supporting patterning means, the patterning means serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate; and
a projection system for projecting the patterned beam onto a target portion of the substrate.
The term xe2x80x9cpatterning meansxe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using such a lithographic projection apparatus, a pattern in a mask (or other patterning means) is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an integrated circuit (IC). Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes may be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997 ISBN 0-07067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
The current state of IC fabrication requires lithography processes to provide for patterning feature line widths to near one-half of the exposure wavelength. For the 150 nm device generation, the KrF excimer laser (DUV; 248 nm) is typically selected as the exposure source of choice. Recent research and development efforts have further demonstrated the possibility of utilizing the KrF excimer laser for the 130 nm device generation. This is achieved inter alia by employing multiple resolution enhancement techniques (RET), such as attenuated phase-shifting masks (attPSM) and off-axis illumination (OAI), in combination with optical proximity correction (OPC) techniques. Possible alternatives to the foregoing techniques are to use a shorter exposure wavelength, such as an ArF excimer laser with a wavelength of 193 nm, or to use a lens with a super-high Numerical Aperture (NA), e.g. NA=0.8 or more. However, both these alternatives require extensive capital expenditure in new apparatus, and it is generally desirable to postpone such expenditure if at all feasible. Consequently, integrated device manufacturers would generally like to get the most out of existing DUV systems before making the switch to successor apparatus.
Regardless of the excimer laser utilized in the fabrication process, the fabrication of devices having critical dimensions of 150 nm or less requires that the near-diffraction-limited lens utilized in the fabrication process be substantially aberration free. As is known, aberrations can be caused by various sources, such as a defective lens or an aging laser that emits a beam having a frequency shifted from the desired value. Accordingly, it is desirable to verify lens performance (i.e. qualify the lens) prior to installation, and then to subsequently monitor the lens performance during use (e.g. in an IC fabrication process).
During the lens manufacturing process, the lens performance can be fully tested interferometrically. Typically, the lens is first qualified at the factory and then again during the initial installation in the field. One common practice utilized for lens qualification is to print wafers and then measure the dimensions of the minimum feature width, or the Critical Dimension (CD). During this qualification process, both xe2x80x9cverticalxe2x80x9d and xe2x80x9chorizontalxe2x80x9d features are measured (i.e. features extending in two orthogonal directions on the substrate plane, such as along X and Y axes). In some instances, the CD for 45-degree features is also measured. In order to verify lens performance, a sufficient number of CD measurements is required across the entire exposure field. The results of the CD measurements are then analyzed to determine whether or not the lens performance is acceptable.
Although the CD measurement method provides a method of evaluating the performance of the lens, it is not a simple task to correlate the CD data to the xe2x80x9csignaturexe2x80x9d of the lens aberration. Accordingly, there have been efforts to perform a direct observation of lens aberrations. For example, an article by Toh et al. entitled xe2x80x9cIdentifying and Monitoring of Lens Aberrations in Projection Printing,xe2x80x9d SPIE Vol. 772, pp. 202-209 (1987) described methods for measuring the effects of relatively large lens aberrations of about 0.2xcex, where xcex is the exposure wavelength. However, for today""s near-diffraction-limited optics, any lens aberration is likely to be in the neighborhood of 0.05xcex, or smaller. For 130 nm features, a 0.05xcex lens aberration translates to a 12.4 nm dimensional error when utilizing the KrF exposure source. Accordingly, if the feature CD budget (i.e. error tolerance) is assumed to be xc2x110% of the target feature width, a 12.4 nm error consumes almost the entire CD budget.
In an article by Gortych et al. entitled xe2x80x9cEffects of Higher-Order Aberrations on the Process Window,xe2x80x9d SPIE Vol. 1463, pp. 368-381 (1991) it was demonstrated that higher-order lens aberrations could deteriorate lithographic process windows. Unfortunately, the higher-order lens aberrations are difficult to eliminate after the photolithography system is assembled. In an article by Brunner entitled xe2x80x9cImpact of Lens Aberration on Optical Lithography,xe2x80x9d INTERFACE 1996 Proceedings, pp. 1-27 (1996) simulation was utilized to demonstrate the negative impact of near-wavelength features due to several first-order lens aberrations. Specifically, it was possible to observe coma aberrations by examining how the contact features were printed when utilizing an attenuated PSM. It is also known that lens aberrations can be balanced with custom off-axis illumination. Others have attempted to make direct measurements of various kinds of lens aberrations in an effort to achieve better CD control.
An article by Farrar et al. entitled xe2x80x9cMeasurement of Lens Aberrations Using an In-Situ Interferometer Reticle,xe2x80x9d Advanced Reticle Symposium, San Jose, Calif. (Jun. 1999) reported the use of an in-situ interferometer reticle to directly measure lens aberration. According to Farrar, it was possible to derive lens aberrations up to 37 Zernike terms. Although Farrar claims that the method is accurate and repeatable, it involves hundreds or thousands of registration type measurements (i.e. the measuring of the shift in relation to the intended feature position). As such, while Farrar""s method may be accurate and repeatable, with the need for such exhaustive measurements, the method is clearly very time consuming, and therefore likely unusable in a manufacturing-driven environment. Furthermore, it is conceivable that minute lens aberrations can drift over time for various reasons (e.g. as a result of the periodic preventive maintenance performed on a system). Thus, as it is critical to monitor lens performance on a periodic basis, the use of Farrar""s method, which requires substantial measurements and calculations, is impractical. Accordingly, there is a need to be able to monitor the lens aberration directly from the printed product wafers.
In an effort to accomplish this objective, Dirksen et al. (see, for example, PCT Patent Application WO 00/31592) proposed a method for directly monitoring lens aberration from the printed wafers. According to Dirksen""s method, the lens monitor comprises simple circular features on the reticle. More specifically, the circular feature is a chromeless feature that has been etched into the vitreous substrate of the reticle. The etched depth is typically xcex/2 and the diameter is about (xcex/NA), where NA is the numerical aperture of the projection lens. According to Dirksen, the method has proven to be effective. Further, the structure is simple and small enough to be readily placed throughout the entire exposure field.
Still, there are a number of issues concerning the use of Dirksen""s lens aberration monitor. First, the depth of the lens monitor feature on the mask needs to be etched to approximately half of the wavelength. For a special-purpose mask, there is no problem dedicating an extra (or special) mask making process step to fabricate such a feature. However, for production reticle types, such as a binary chrome reticle or attPSM, an extra mask making process step necessary to create such a monitor is a costly and time-consuming process. Alternating PSM (altPSM) or chromeless PSM (CLM) would also require the extra mask making process step. Further, since the Dirksen monitor calls for a different etch depth in the quartz substrate as opposed to the xcfx80-phase, it requires a special etch time and must be done separately.
A second issue with Dirksen""s lens monitor is that it is vulnerable to phase error that may result from the quartz etch process during mask formation. More specifically, referring to FIGS. 1(a)-1(f) (in which S denotes a quartz mask substrate), for an exacerbated phase error, the quartz etch process causes a sloped edge profile on the mask as shown in FIG. 1(a). In such a case, the Dirksen monitor loses all of the sensitivity to indicate any possible lens aberration. However, when there is no phase-error on the mask, as shown in FIG. 1(d), the Dirksen monitor is effective for detecting lens aberrations. FIGS. 1(b) and 1(e) illustrate a cross-sectional view of the printed resist pattern resulting from the xe2x80x9cslopedxe2x80x9d Dirksen monitor structure of FIG. 1(a) and the xe2x80x9cidealxe2x80x9d Dirksen monitor structure of FIG. 1(d), respectively.
It is noted that the printing conditions utilized to produce the resist profiles illustrated in FIGS. 1(b) and 1(e) were as follows: a 0.68NA with 0.8 partial coherence at +0.1 xcexcm de-focus, utilizing a Shipley UV6 resist with a thickness of 0.4 xcexcm on an organic BARC (AR2) on top of a polysilicon wafer. The simulation introduced a +0.025xcex coma for both X and Y (Z7 and Z8 Zernike terms).
Upon a closer examination of the ring-shaped resist patterns formed by the Dirksen monitor structures, as shown for example in FIGS. 1(c) and 1(f), it is clear that the inner ring of the printed resist pattern has a relatively sloppy resist profile in contrast to the steep profile formed by the outer ring structure. The reason for this variation is that the outer-ring resist pattern is formed by the phase change in the mask, while the inner ring resist pattern is formed without any such phase change. Specifically, the inner ring resist pattern is formed via the attenuation of the exposure wavelength that is passed through the center of the Dirksen monitor pattern. In other words, the two resist profiles (i.e. the inner ring and the outer ring) are formed by two inherently different log-slopes of the respective aerial images. The difference in resist profiles can lead to erroneous registration measurements, which can cause a misinterpretation of the lens aberration in question.
It is noted that it is possible to observe a slight coma with the Dirksen lens aberration monitor, as shown in FIGS. 1(e) and (f). Specifically, the width of the ring is different on the left side as compared to the right side. It is further noted that it is difficult to observe this coma in the xe2x80x9cslopedxe2x80x9d Dirksen monitor, as shown in FIGS. 1(b) and 1(c).
Accordingly, in view of the foregoing problems, there remains a need for a lens monitor that allows for the detection of lens aberrations, but which is not easily impaired by slight imperfections in the mask making process. It is also desirable that the lens monitor structures be small enough such that they can be positioned in numerous places between or beside production die for in-situ lens monitoring purposes. It is also desirable that the lens monitor can be fabricated without requiring extra mask making process steps.
In an effort to solve the aforementioned needs, it is an object of the present invention to provide a lens monitor capable of detecting lens aberrations. It is a further object of the invention that said lens monitor comprise aberration analysis structures of sufficiently small size to allow the monitor to be utilized for in-situ production monitoring. Moreover, it is an object of the invention that the manufacture of said monitor should not require extra processing steps, e.g. during mask formation, and that the functionality of the lens monitor should not be significantly impaired by minor imperfections in its manufacture, e.g. in the mask formation process.
More specifically, the present invention relates to a lens aberration monitor for detecting lens aberrations. The monitor comprises a plurality of non-resolvable features (disposed, for example, on a mask). The plurality of non-resolvable features is arranged so as to project a predetermined test pattern on the substrate, which test pattern is then utilized to detect lens aberrations. The size of the monitor is such as to fit within the object field of the lithographic apparatus in conjunction with a device pattern, corresponding to a device (e.g. an integrated circuit) to be formed on the substrate; for example, the monitor is small enough to fit on a mask containing an IC pattern.
The present invention also relates to a method of detecting aberrations associated with an optical system (radiation system and/or projection lens) utilized in an optical lithography system as specified in the opening paragraph. In that context, the method comprises the steps of:
providing said desired pattern to comprise a monitor having a plurality of non-resolvable features, where the plurality of non-resolvable features is arranged so as to form a predetermined test pattern when projected on the substrate;
projecting the monitor onto the substrate using the projection system, and;
analyzing the position of said predetermined test pattern and the position of the plurality of non-resolvable features in the monitor so as to determine if there is an aberration.
In addition to said monitor, said desired pattern may further comprise a device pattern, corresponding to an integrated device layer to be formed on said substrate.
As explained below, if the position of the predetermined test pattern differs from an expected position, which is determined from the position of the plurality of non-resolvable features, this shift from the expected position indicates the presence of an aberration.
As described in further detail below, the present invention provides significant advantages over the prior art. Most importantly, the present invention provides a lens monitor capable of detecting very subtle lens aberrations. Furthermore, as the overall size of the lens monitor structures is sufficiently small, the monitor structures can be positioned in a sufficient number of positions in so as to allow for monitoring of the entire exposure field.
If the monitor is disposed on a mask, then it is substantially immune to deficiencies in the mask formation process utilized to form the monitor. In such a case, the lens monitor of the present invention is suitable for in-situ monitoring, as the lens monitor can be formed utilizing the same mask formation process required to form the production mask, and therefore does not require any additional mask formation processing steps. Yet another advantage is that the effectiveness of the lens monitor is relatively insensitive to both the xe2x80x9cslopedxe2x80x9d phase edges and the xe2x80x9ccorner roundingxe2x80x9d effects that are generally inherent to the mask making process.